There are many patents dealing with process for producing ferrous picrate fuel additives.
These include U.S. Pat. Nos. 2,506,539; 3,282,858; 4,073,626; 4,099,930; 4,129,421; 4,265,639; 4,424,063; 5,087,268; 5,359,103; 5,720,783; and 5,925,153.
Only U.S. Pat. Nos. 5,087,268 and 5,925,153 employ metallic iron; and these both utilize powdered elemental iron. The large surface area of powdered elemental iron facilitates the desired reaction.